The present application claims priority to Japanese Application No. P2000-24 1787 filed Aug. 9, 2000, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a laser beam-generating apparatus which performs wavelength conversion on the input beam by means of a nonlinear optical element and which emits a laser beam of a specific wavelength.
2. Description of the Related Art
A technique has been known in which an optical resonator performs wavelength conversion on the input beam, thus generating the second harmonic of the fundamental wave. Another technique is known, in which beams of different wavelengths are mixed, thereby emitting a laser beam of a desired wavelength. (See A. Ashkin, G. D. Boyd, and J. M. Dziedzic, xe2x80x9cResonant Optical Second Harmonic Generation and Mixing,xe2x80x9d IEEE J. Quant. Electron. Vol. QE-2, pp. 109-124, 1966.)
A laser beam-generating apparatus, which utilizes such a technique to emit a deep-ultraviolet laser beam having the wavelength of 266 nm, has been put to practical use. The laser beam-generating apparatus uses a green laser beam as the fundamental wave. It comprises an optical resonator having a nonlinear optical element, which is a BBO (xcex2-barium borate: xcex2-BaB2O4) crystal. The green laser beam is applied to the optical resonator and resonated in the optical resonator, thereby generating a deep-ultraviolet laser beam, i.e., the second harmonic. The laser beam-generating apparatus is used as an efficient light source in, for example, laser microscopes.
The laser beam-generating apparatus comprises an optical resonator that changes the wavelength of the input beam. The optical resonator has an optical energy loss. Even if the optical energy loss increases only a little, the efficiency of wavelength conversion will greatly decrease. Consequently, the output of the optical resonator will proportionally decrease. To achieve an effective use of the laser beam-generating apparatus, it would be important to reduce the increase in the optical energy loss in the resonator, thereby to enhance the reliability of the laser beam-generating apparatus.
To reduce the increase in the optical energy loss in any optical resonator, it is important to prevent dirt from sticking to the surfaces of the optical components that constitute the optical resonator. It is known that the dirt on each optical component is a deposit of the impurities in the air, such as ammonium sulfate, formed on that surface of the component which is irradiated with the second harmonic (i.e., deep-ultraviolet laser beam). How the impurities in the air deposit on the optical components remains unclear. Nonetheless, the dirt is considered to have resulted from chemical reaction caused by the deep-ultraviolet laser beam. This is because the impurities deposit on only that part of each component that is irradiated with the deep-ultraviolet laser beam.
The optical resonator incorporated in the above-mentioned laser beam-generating apparatus has a BBO crystal as the nonlinear optical element. The BBO crystal, which is an almost rectangular flat plate, is used because it is easy to process. The green laser beam, or the fundamental wave, is applied to the BBO crystal. In the BBO crystal, the green laser beam passes through an optical path that satisfies the conditions for phase matching. The laser beam immediately emerges from the BBO crystal. As the green laser beam passes through the optical path in the BBO crystal, the second harmonic, i.e., deep-ultraviolet laser beam, is generated from the green laser beam. The deep-ultraviolet beam emerges from the BBO crystal and travels in almost the same optical axis as the green laser beam (i.e., the fundamental wave) does.
The green laser beam and the deep-ultraviolet laser beam, both travelling from the BBO crystal, are applied to a reflector located on their common optical axis. The reflector has a high reflectance to the green laser beam (i.e., fundamental wave) and a high transmittance to the deep-ultraviolet laser beam (i.e., second harmonic). The reflector reflects the green laser beam, which is further reflected by other reflectors and applied back into the BBO crystal. On the other hand, the deep-ultraviolet laser beam passes through the reflector and ultimately emerges from the optical resonator.
Impurities, such as ammonium sulfate, may deposit on that part of the reflector through which the deep-ultraviolet laser beam passes, due to the chemical reaction caused by the deep-ultraviolet laser beam. (Note that this reflector is provided on the common optical axis of the green laser beam and the deep-ultraviolet laser beam, both travelling from the BBO crystal.) Such deposition of impurities results in the decrease of the energy of the fundamental wave, i.e., green laser beam, because the green laser beam is reflected at the said part of the reflector.
To prevent the deposition of impurities, the following measures are taken in manufacturing the laser beam-generating apparatus. First, not only the optical components constituting the optical resonator, but also the mechanical parts for supporting the optical components are thoroughly washed, before the optical resonator is assembled. Further, dry, clean air is continuously applied into the housing of the laser beam-generating apparatus after the optical resonator has been set within the housing.
Notwithstanding these measures are taken, the impurities cannot be completely prevented from depositing on the reflector that is provided on the common optical axis of the green laser beam and the deep-ultraviolet laser beam, both travelling from the BBO crystal. The optical energy loss pertaining to the fundamental wave inevitably increases. Consequently, the efficiency of wavelength conversion greatly lowers in some cases, resulting in a decrease in the output of the laser beam-generating apparatus.
The present invention has been made in view of the forgoing. An object of the invention is to provide a laser beam-generating apparatus in which the deposition of impurities on the surfaces of the optical components is effectively inhibited, thus preventing an increase in the optical energy loss pertaining to the fundamental wave, and which can therefore emits a stable, intense laser beam and therefore has high reliability.
A laser beam-generating apparatus according to the invention comprises: wave-generating means, a nonlinear optical element, wave-reflecting means, and harmonic-reflecting means. The wave-generating means generates a fundamental wave. The nonlinear optical element receives the fundamental wave generated by the wave-generating means and allows the fundamental wave to pass along an optical path satisfying conditions for phase matching, thereby to generate a harmonic wave. The nonlinear optical element has a plurality of internal total reflection planes. It emits the fundamental wave sequentially reflected at the internal total reflection planes, in a direction intersecting with an axis of the fundamental wave applied to the nonlinear optical element. Further, it emits the harmonic wave sequentially reflected at the internal total reflection planes, spatially deviated from the fundamental wave, by utilizing birefringence in the nonlinear optical element and the internal total reflections at the internal total reflection planes. The wave-reflecting means is provided at an intersection of the axis of the fundamental wave applied to the nonlinear optical element and an axis of the fundamental wave emitted from the nonlinear optical element. The wave-reflecting means reflects the fundamental wave emitted from the nonlinear optical element and applies the same again into the nonlinear optical element. The harmonic-reflecting means reflects the harmonic wave emitted from the nonlinear optical element and spatially deviated from the fundamental wave, thereby to emit the harmonic wave from the laser beam-generating apparatus.
In the laser beam-generating apparatus, the wave-generating means generates a fundamental wave. The fundamental wave is applied to the nonlinear optical element. In the element, the fundamental wave is sequentially reflected at the internal total reflection planes. The fundamental wave thus reflected is emitted from the nonlinear optical element, with its axis intersecting with the axis of the fundamental wave being applied to the nonlinear optical element. The wave-reflecting means is provided at the intersection of the axes of the fundamental waves applied to and emitted from the nonlinear optical element, respectively. It reflects the fundamental wave emitted from the nonlinear optical element. Thus reflected, the fundamental wave is applied to the nonlinear optical element again. A closed optical path is therefore formed for the fundamental wave. Thus, an optical resonator is provided.
As the fundamental wave travels in the nonlinear optical element, along the optical path satisfying conditions for phase matching, a harmonic wave is generated from the fundamental wave. The harmonic wave is sequentially reflected at the internal total reflection planes and is emitted from the nonlinear optical element. It is spatially deviated from the fundamental wave, due to the birefringence in the nonlinear optical element and the internal total reflections at the internal total reflection planes. The harmonic-reflecting means reflects the harmonic wave emitted from the nonlinear optical element. The harmonic wave is then emitted from the laser beam-generating apparatus.
Since the fundamental wave and the harmonic wave, both emitted from the nonlinear optical element, are spatially separated from each other in the laser beam-generating apparatus, it is possible not to apply the harmonic wave to the wave-reflecting means which reflects the fundamental wave. Therefore, no chemical reaction takes place, and no impurities, which may be generated by such reaction, deposit on the means for reflecting the fundamental wave. This reliably prevents an optical energy loss at the wave-reflecting means.
In a laser beam-generating apparatus according to the present invention, the nonlinear optical element generates and emits a harmonic, which is spatially deviated from the fundamental wave applied to the nonlinear optical element. It is therefore possible not to apply the harmonic wave to the wave-reflecting means. No chemical reaction takes place, which might occur if the harmonic were applied to the wave-reflecting means. No impurities, which may be generated by such reaction, deposit on the wave-reflecting means. Hence, the laser beam-generating apparatus can attain high wavelength conversion efficiency.